1. Field of the Invention
The present invention relates to a method for determining a pattern for modulating the power of a laser beam, an apparatus for recording data in an optical recording medium and an optical recording medium, and particularly, to a method for determining a pattern for modulating the power of a laser beam which can determine a pattern for modulating the power of a laser beam so that data can be recorded in a write-once type optical recording medium with a laser beam having a low recording power at a high linear recording velocity, an apparatus for recording data in an optical recording medium which can determine a pattern for modulating the power of a laser beam so that data can be recorded in a write-once type optical recording medium with a laser beam having a low recording power at a high linear recording velocity, and a write-once type optical recording medium which can determine a pattern for modulating the power of a laser beam so that data can be recorded therein with a laser beam having a low recording power at a high linear recording velocity.
2. Description of the Related Art
Optical recording media such as the CD, DVD and the like have been widely used as recording media for recording digital data. These optical recording media can be roughly classified into optical recording media such as the CD-ROM and the DVD-ROM that do not enable writing and rewriting of data (ROM type optical recording media), optical recording media such as the CD-R and DVD-R that enable writing but not rewriting of data (write-once type optical recording media), and optical recording media such as the CD-RW and DVD-RW that enable rewriting of data (data rewritable type optical recording media).
As well known in the art, data are generally recorded in a ROM type optical recording medium using pre-pits formed in a substrate in the manufacturing process thereof, while in a data rewritable type optical recording medium a phase change material is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by phase change of the phase change material.
On the other hand, in a write-once type optical recording medium, an organic dye such as a cyanine dye, phthalocyanine dye or azo dye is generally used as the material of the recording layer and data are recorded utilizing changes in an optical characteristic caused by chemical change of the organic dye, or chemical change and physical change of the organic dye.
Further, there is known a write-once type recording medium formed by laminating two recording layers (See Japanese Patent Application Laid Open No. 62-204442, for example) and in this optical recording medium, data are recorded therein by projecting a laser beam thereon and mixing elements contained in the two recording layers to form a region whose optical characteristic differs from those of regions therearound.
In this specification, in the case where an optical recording medium includes a recording layer containing an organic dye, a region in which an organic dye chemically changes or chemically and physically changes upon being irradiated with a laser beam is referred to as “a recording mark” and in the case where an optical recording medium includes two recording layers each containing an inorganic element as a primary component, a region in which the inorganic elements contained in the two recording layers as a primary component are mixed upon being irradiated with a laser beam is referred to as “a recording mark”.
An optimum method for modulating the power of a laser beam projected onto an optical recording medium for recording data therein is generally called “a pulse train pattern” or “recording strategy”.
FIG. 14 is a diagram showing a typical pulse train pattern used for recording data in a CD-R including a recording layer containing an organic dye and shows a pulse train pattern for recording 3T to 11T signals in the EFM Modulation Code.
As shown in FIG. 14, in the case where data are to be recorded in a CD-R, a recording pulse having a width corresponding to the length of a recording mark M to be formed is generally employed (See Japanese Patent Application Laid Open No. 2000-187842, for example).
More specifically, the power of a laser beam is fixed at a bottom power Pb when the laser beam is projected onto a blank region in which no recording mark M is formed and fixed at a recording power Pw when the laser beam is projected onto a region in which a recording mark M is to be formed. As a result, an organic dye contained in a recording layer is decomposed or degraded at a region in which a recording mark M is to be formed and the region is physically deformed, thereby forming a recording mark M therein. In this specification, such a pulse train pattern is called a single pulse pattern.
FIG. 15 is a diagram showing a typical pulse train pattern used for recording data in a DVD-R including a recording layer containing an organic dye and shows a pulse train pattern for recording a 7T signal in the 8/16 Modulation Code.
Since data are recorded in a DVD-R at a higher linear recording velocity than when recording data in a CD-R, unlike the case of recording data in a CD-R, it is difficult to form a recording mark having a good shape using a recording pulse having a width corresponding to the length of the recording mark M to be formed.
Therefore, data are recorded in a DVD-R using a pulse train in which, as shown in FIG. 9, the recording pulse is divided into a number of divided pulses corresponding to the length of the recording mark M to be formed.
More specifically, in the case of recording an nT signal where n is an integer equal to or larger than 3 and equal to or smaller than 11 or 14 in the 8/16 Modulation Code, (n−2) divided pulses are employed and the power of the laser beam is set to a recording power Pw at the peak of each of the divided pulses and set to a bottom power Pb at the other portions of the pulse. In this specification, the thus constituted pulse train pattern is referred to as “a basic pulse train pattern”.
As shown in FIG. 9, in the basic pulse train pattern, the level of a bottom power Pb is set to be equal to a reproducing power Pr used for reproducing data or close thereto.
On the other hand, a next-generation type optical recording medium that offers improved recording density and has an extremely high data transfer rate has been recently proposed.
In such a next-generation type optical recording medium, in order to achieve an extremely high data transfer rate, it is required to record data at a higher linear recording velocity than that in a conventional optical recording medium, and since the recording power Pw necessary for forming a recording mark is generally substantially proportional to the square root of the linear recording velocity in a write-once optical recording medium, it is necessary to employ a semiconductor laser having a high output for recording data in a next-generation optical recording medium.
Further, in the next-generation type optical recording medium, the achievement of increased recording capacity and extremely high data transfer rate inevitably requires the diameter of the laser beam spot used to record and reproduce data to be reduced to a very small size.
In order to reduce the laser beam spot diameter, the numerical aperture of the objective lens for condensing the laser beam needs to be increased to 0.7 or more, for example, to about 0.85, and the wavelength of the laser beam needs to be shortened to 450 nm or less, for example, to about 400 nm.
However, the output of a semiconductor laser emitting a laser beam having a wavelength equal to or shorter than 450 nm is smaller than that of a semiconductor laser emitting a laser beam having a wavelength of 780 nm for a CD and that of a semiconductor laser emitting a laser beam having a wavelength of 650 nm for a DVD, and a semiconductor laser that emits a laser beam having a wavelength equal to or shorter than 450 nm and has a high output is expensive.
Therefore, it is difficult to record data in a next-generation type optical recording medium at a high data transfer rate using the basic pulse train pattern.
The above described problems are particularly serious in a write-once type optical recording medium which is constituted so that a recording mark is formed by projecting a laser beam thereonto and mixing elements contained in a plurality of recording layers by heat generated by the laser beam.